This invention relates to radio communications systems receivers and more specifically to a radio receiver squelch circuit.
Squelch circuits have long been used in radio communications receivers. A squelch circuit causes the audio output signal coming from a loudspeaker or headphones to be muted or turned off when there is no modulated voice signal. When a desired voice signal appears in the radio receiver, the squelch circuit turns on the audio output signal to the loudspeaker or headphones. The squelch circuit serves to turn off the audio output when no desired signal is present to eliminate the noise and other interference on a communications channel when no signal is present to reduce radio operator fatigue.
Many different types of squelch circuits have been developed. The simplest squelch circuits include those that sense the presence of a signal carrier in the radio receiver by sensing the automatic gain control (AGC) voltage level and comparing it to an adjustable threshold level. These squelch circuits often trigger on signals other than voice such as a burst of noise or an unmodulated carrier wave (CW). This turning on or triggering of the audio output in the presence of undesired signals has resulted in the development of alternate squelch circuits.
An alternate form of squelch circuit in use on amplitude modulated (AM) radios is the AM quieting noise squelch circuit. In this type of squelch circuit, signal presence is sensed by comparing the signals from the AM detector in a low-frequency band-pass channel to a high-frequency band-pass channel. With no signal, noise in the high-frequency channel exceeds the signal in the low-frequency channel. When a signal is present in the radio, the amount of noise is reduced in the high-frequency channel due to AGC and the signal in the low-frequency channel increases to exceed the high-frequency channel. The audio is then turned on. A noise quieting type squelch circuit can be falsely triggered by an undesired signal such as any CW signal in the receiver pass band.
To overcome squelch triggering by CW signals, syllabic filters have been added to the low-frequency channel to sense the syllabic or very low-frequency component in speech. This feature, along with others such as automatic level control, is disclosed in U.S. Pat. No. 4,414,689 incorporated herein by reference and hereinafter referred to as the ""689 patent. The basic principle of the squelch circuit of the ""689 patent is that the noise band channel looks for a noise sample whereas the speech detector looks for syllabic rate variations or speech. The noise sample or noise band channel will have low energy when there is speech due to functioning of the AGC and the voice or speech detector will have high energy due to the speech. This concept works in conventional applications and also gives a certain amount of rejection of CW signals in the output of the AM detector as long as the interfering signals do not fall in the bandwidth (BW) of either channel band-pass filter.
What is needed is the capability to not trigger the squelch circuit when several CW interferers are present in the intermediate frequency (IF) or baseband channel of the radio. These CW interferers may be much stronger than a desired signal. The squelch circuit should trigger with all levels of desired input signals with or without the undesired CW interferers that may be of greater or lower signal strength than the desired signal.
A multiband squelch circuit for a radio receiver to detect the presence of speech and enable an audio output device is disclosed. A filter bank with a group of band-pass filters or a polyphase filter for filtering an intermediate frequency or a baseband signal from the radio receiver into a plurality of band-pass signals is employed. The magnitudes of the band-pass signals received from the filter bank are determined in a magnitude detector function. The magnitudes of the band-pass signals are passed to a speech detector where a signal averager determines the average values of the band-pass signal magnitudes. A syllabic filter filters the average values to obtain the voice band average signals and a second filter further filters the voice band average signals from the syllabic filter. A syllabic differencer determines the difference signals between the voice band average signals and the second filter outputs. The difference signals are indicative of a syllabic rate of speech when the voice band average signals are greater than the second filter outputs. A maximum value selector selects a maximum difference signal that indicates there is speech present in the speech detector and that a speech detector decision is made. A commutator selects the voice band average signal that is associated with the maximum difference signal. The magnitudes of the band-pass signals are also passed to a noise detector where a peak detector determines the peak values of the signal magnitudes. The peak values are then filtered by an unsymmetrical low-pass filter to obtain noise detector signals. A minimum value selector selects a lowest noise detector signal from the noise detector signals. A threshold function scales the voice band average signal associated with the maximum difference signal and a hilo differencer determines a hilo difference signal between the lowest noise detector signal and the scaled voice band average signal associated with the maximum difference signal. The hilo difference signal is indicative of the presence of speech and a noise detector decision is made when the lowest noise detector signal is less than the scaled voice band average signal. An AND logic function determines the final decision signal to enable the audio output device when the speech detector decision and the noise detector decision both indicate speech. A hang counter passes the final decision signal to enable the audio output device and to hold the audio device on for a predetermined hold time to prevent on/off triggering of the audio.
It is an object of the present invention to provide a squelch circuit that does not trigger with several CW interferers in the intermediate frequency (IF) or baseband channel of the radio when the CW interferers are much stronger than the desired signal when it is present.
It is another object of the present invention to provide a squelch circuit that triggers with all levels of desired input signals with or without the undesired CW interferers that may be of greater or lower signal strength than the desired signal.
It is an advantage of the present invention to provide a squelch circuit that can be implemented using digital signal processing techniques.
It is a feature of the present invention to implement the squelch circuit in a polyphase filter implementation requiring reduced signal processing resources.
These and other objects, features, and advantages are disclosed and claimed in the specification, figures, and claims of the present application.